1. Field of the Invention
The invention relates to electronic communication and, more particularly, to the carrier-recovery loop of a radio receiver.
2. Description of the Related Art
Radio communication is generally accomplished through a carrier wave that is modulated to bear information from a transmitting unit to a receiving unit. The transmission of the information involves modulating the carrier wave with a baseband signal that represents the information to be transmitted. Typically, the carrier wave is generated by an oscillator in the transmitter and modulated by a modulator to produce the transmitted signal. After traversing a communication channel, this signal is received by the receiver that demodulates it to extract the baseband signal.
An important component of the receiver is a recovery oscillator used in demodulating the received signal. The recovery oscillator generates a recovered signal, at a recovered frequency, that is used to demodulate the received signal. In communications systems that use digital modulation techniques such as differential phase-shift keying, the recovered frequency must be close to the frequency of the carrier wave: if these two frequencies are not matched, the receiver cannot efficiently demodulate the transmitted signal.
The receiver's recovery oscillator can be designed so that its natural frequency is close to that of the transmitter oscillator, but due to variations in manufacturing and differences in operating environments there will be frequency differences between the two oscillators. To compensate for the resulting offsets in frequency between the carrier wave and the receiver's recovery oscillator, the recovery oscillator can be locked to the carrier wave by incorporating it into phase-locked loop (PLL). Such a PLL serves as a carrier-recovery loop that ties the frequency of the recovery oscillator to the frequency of the carrier wave.
In addition to the recovery oscillator, the carrier-recovery loop includes a phase detector and a loop filter. The phase detector generates an error signal that represents the difference in phase between the recovery oscillator and the carrier wave. Since the original carrier wave is not typically available to the receiver unit, the phase detector must be able to extract the frequency of the carrier wave from the received signal. That is, it must be able to ignore variations in the received signal's phase that are due to the information encoded onto the carrier. For example, in the case of digital communication systems that use quadriphase-shift-keying (QPSK) modulation, information is encoded on the carrier by changing its phase by multiples of 90.degree.. In these systems, the recovery loop's phase detector must not interpret these deliberate phase shifts of the carrier as erroneous frequency drifts in the carrier. Depending on the type of modulation, there are several established methods of making the phase detector in the carrier-recovery loop insensitive to information-bearing phase shifts on the carrier wave, while keeping its sensitivity to frequency drifts in the carrier.
The loop filter in the carrier-recovery loop receives the error signal from the phase detector. The error signal is conditioned (typically by low-pass filtering) in the loop filter to generate a feedback signal, which is then fed back to the recovery oscillator to keep its frequency matched to the frequency of the carrier wave.
The desired condition of the PLL, in which the recovery oscillator matches the frequency of the carrier wave, is a phase lock. Under this condition the carrier-recovery loop holds the phase of the recovered signal at a constant offset from the phase of the carrier wave, thereby matching the carrier's frequency. There is an inherent delay from the time that the received signal is provided to the carrier-recovery loop to the time that the carrier-recovery loop acquires a phase lock. This delay is the loop's acquisition time, which depends on the initial difference in frequency between the carrier wave and the recovery oscillator, on the response characteristics of the carrier-recovery loop, on the stability of the carrier wave, and on the presence of noise and interference in the received signal. During this acquisition time, the receiver cannot efficiently demodulate the received signal. Hence it is desirable to have a carrier-recovery loop with a reduced acquisition time.